1. Field of the Invention
The present invention relates generally to sealing systems for thermoplastic pipes and, more specifically, to an improved pipe gasket and to an improved belling process for installing a gasket in a socket end of a thermoplastic pipe.
2. Description of the Prior Art
Pipes formed from thermoplastic materials including polyethylene and PVC are used in a variety of industries. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. It is critical, during the installation process, that the gasket not be able to twist, flip or be displaced, since a displaced or dislocated gasket will adversely affect the ultimate sealing capacity of the joint.
In the early 1970's, a new technology was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the “Rieber Joint.” The Rieber system employed a combined mold element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, the elastomeric gasket was inserted within an internal groove in the socket end of the female pipe as the female or belling end was simultaneously being formed. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint. These features increased the reliability of the joint and decreased the risk of leaks or possible failure due to abrasion or other factors. The Rieber process is described in the following issued U.S. Pat. Nos. 4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,887,992; 3,884,612; and 3,776,682.
In the Rieber process, the gasket is installed in a circumferential groove provided upon the working surface of a mandrel and abuts a backup collar which helps to position and retain the gasket during the subsequent belling operation. The associated thermoplastic pipe was then heated and the heated thermoplastic pipe end was forced over the mandrel and gasket. The pipe socket end was deformed by the gasket and an internal retention groove was formed in the interior of the pipe end. The pipe end was then cooled so that it would retain its shape and the mandrel was retracted, leaving the sealing gasket fixed within the retention groove.
In the Rieber process, the gasket always included an internal metal reinforcing component which circumscribed the gasket body and gave it additional rigidity. The metal component, either a band or wire, was bonded to the rubber and acted as the structural member to keep the gasket engaged in the pipe socket after the belling operation was complete. As such, although the pipe wall was formed over the Rieber gasket, the two components were discrete.
Other commercial gaskets available in the industry, for example the Forsheda POWER LOCK® and the Vassallo EPSMI®, did not utilize a bonded metal component, but instead used a hard polymer component for the trailing region (ramp). The hard polymer component acted in the same structural manner as the metal component in the original Rieber belling operation. As such, the hard polymer portion of the gasket was necessarily heavy with a relatively thick cross section. The pipe wall was formed over the gasket and these two components again remained discrete in the final assembly. In those cases where material was removed from the trailing region of the gasket, as in the Vassallo gasket, it was removed to reduce mass while retaining structural integrity.
A need exists, therefore, for an improved sealing gasket for use in a Rieber type manufacturing process, which sealing gasket would be even more positively and “integrally locked” within the retention groove of the socket pipe end during the belling operation.
A need also exists for such a sealing gasket which would be simple in design and simple to manufacture and which could also be used without the necessity of modifying the design of existing belling machines.
A need also exists for such a sealing gasket which, because of its composite construction, would eliminate the need for an internal reinforcing ring, thereby reducing the cost of the gasket and simplifying the belling operation and improving the cycle time of the manufacturing operation.
A need also exists for such a sealing gasket which, due to its composition, would eliminate the need for the use of a lubricant during the belling operation.
A need also exists for such a gasket which could be color coded so that, for example, the gasket could be coded according to size or end use.
A need also exists for such a gasket which, due to its composition, could utilize recycled scrap, thereby eliminating much of the scrap waste present in conventional belling operations.